(1) Field of the Invention
The present invention relates to a method used to create metal interconnect structures, for semiconductor devices, and more specifically to a method used to pattern thin copper layers, used for metal interconnect structures.
(2) Description of Prior Art
The objectives of the semiconductor industry are to continually improve performance of semiconductor devices, while still attempting to decrease the cost of these same semiconductor devices. Micro-miniaturization, or the ability to create semiconductor devices with sub-micron features, have allowed these performance and cost objectives to be successfully addressed. For example smaller features result in a decrease in performance degrading capacitances and resistances, for device regions in the semiconductor substrate. In addition the use of sub-micron features, allow smaller semiconductor chips to be realized, however still possessing device densities, comparable to densities achieved with larger semiconductor chip counterparts. This allows more chips to be realized from a specific size starting substrate, thus reducing the processing cost for a specific semiconductor chip.
One area in which micro-miniaturization has in some respects adversely influenced performance, is the creation of sub-micron, metal interconnect structures. Although the resistance of metal interconnect structures is reduced by the decreasing lengths of the metal runs, a reduction in the width of the metal lines, and of the thickness of the metal lines, necessitated in order to reduce topography, results in resistance increases for the narrower, thinner metal lines, as well as increased risk of electromigration failures, due to the higher current densities carried by the narrower, thinner metal lines. One solution for higher resistance, and higher electromigration risk, for thinner metal interconnect structures, is the use of lower resistivity, copper interconnect structures, in place of the higher resistivity, aluminum based, interconnect structures. The use of copper, in addition to offering lower sheet resistance, than aluminum counterparts, also offers increased resistance to electromigration failure, as a result of the increased self-diffusivity of copper, when compared to aluminum counterparts.
The use of copper interconnect structures, however, introduce several new concerns, that were not relevant with the aluminum based metallization. For example the patterning procedure, used to obtain the copper interconnect structures, is accomplished via a reactive ion etching, (RIE), technology, using a photoresist shape as an etch mask, and using a chlorine based chemistry as the etchant gas for the copper layer. However during the etching procedure, using a chlorine etch chemistry, a by-product, such as CuCl.sub.x can be formed, redepositing on regions in which copper still remains unetched, and thus interfering with the patterning procedure. One method used to remove the CuCl.sub.x by-product, during the etch procedure, thus minimizing the effect of the redeposited byproduct, has been the use of higher RIE temperatures, in the range of about 200.degree. C. However the higher RIE temperatures, although removing unwanted by-products via vaporization, or increased RIE bombardment, can result in flowing, or softening of the masking photoresist shapes, resulting in irregular copper shapes.
This invention will offer a solution for the removal of by-products of a chlorine based, RIE procedure, used for copper patterning, performed at RIE temperature in which the integrity of the masking photoresist shapes is not adversely influenced. The addition of nitrogen, in amounts equal to, or greater than, the amount of the chlorine based etchant, results in a non cross-linked by-product, CuCl.sub.x N.sub.y, which is simultaneously, and quickly removed, during the RIE procedure, not interfering with the copper patterning procedure. Prior art, such as Shinohara et al, in U.S. Pat. No. 5,505,322, describe a process for patterning copper, using an etch chemistry comprised of a halogen gas, and nitrogen, however that prior art also features the use of oxygen, as a major etch component. The use of oxygen can deleteriously attack the masking photoresist shape, as well as forming a oxide on the exposed sides of the patterned copper interconnect structure. This present invention accomplishes copper patterning, at low temperatures, however without the use of oxygen additions.